Fiber lasers and amplifiers have a number of advantages over free-space lasers and amplifiers. In fiber lasers and amplifiers, the light is guided by an “active” fiber core doped with a material that provides optical gain, which makes fiber lasers and amplifiers insensitive to mechanical misalignment. The light guiding property of the active optical fiber also allows one to increase the length of the gain medium to tens and even hundreds of meters, resulting in very high achievable optical gains.
There is currently great interest in double-clad fiber lasers and amplifiers and related waveguide devices, thanks to their very high efficiency, brightness, compactness, and cost-effectiveness. Snitzer et al. in U.S. Pat. No. 4,815,079 and Grubb et al. in U.S. Pat. No. 6,157,763 disclose double-cladding optical fiber structures enabling multimode pump radiation from laser diodes to be coupled efficiently into rare-earth-doped single mode cores of optical fibers.
As mentioned in the Snitzer patent, useful glass dopant materials for the double-clad configuration can include neodymium ions (Nd3+) and ytterbium ions (Yb3+), which are superficially similar in that they both amplify light at wavelengths around 1060 nm, and they can both be pumped by near-infrared GaAs laser diodes. In the early 1990's, experiments were done using double-clad Nd3+ lasers, in part because of the availability of laser diodes at the 805 nm pump wavelength of Nd3+ that were developed for pumping Nd:YAG crystal lasers. In the late 1990's, highly reliable laser diodes in the 900-980 nm range became widely available as pumps for telecommunications optical amplifiers. Since this range covers the Yb3+ pump bands, it became practical to use Yb3+ rather than Nd3+ as the lasing dopant in double-clad lasers for operation at the wavelength of about 1060 nm.
Yb3+ has rapidly become the preferred dopant over Nd3+ for operation at 1060 nm because of the following advantages known to the person skilled in the art:
(a) Yb3+ can amplify light more efficiently than Nd3+, because the energy loss between photons at the pump and the output wavelengths is smaller. By way of example, Yb3+ in silica fibers can be pumped at up to 980 nm and amplify light at wavelengths as low as 1030 nm, resulting in a quantum yield of 95%, whereas Nd3+ is typically pumped at 805 nm amplifying light at 1060 nm, resulting in a quantum yield of only 75%.(b) Yb3+ fiber lasers can utilize higher doping levels than Nd3+. Referring to FIG. 1, Yb3+ and Nd3+ energy level structures are shown side-by-side. It is seen that Yb3+ has only two energy-level manifolds, the 5F5/2 and the 5F7/2, which inherently avoids multi-level parasitic processes such as concentration quenching, excited-state absorption, and energy-transfer up-conversion. These parasitic processes can affect Nd3+ having many more energy levels, the 4I9/2 to 4I15/2, 4F5/2, and the 4F3/2. Because these processes are correlated with rare-earth doping concentration, Nd3+ is typically doped at lower concentrations in silica fiber than Yb3+, resulting in reduced fiber lengths as well as further efficiency advantages for Yb3+.(c) Yb3+ has an excited-state lifetime of approximately 1 ms, compared to approximately 0.25 ms for Nd3+, which results in further efficiency advantages for Yb3+.(d) Yb3+ can amplify light across a wider wavelength spectrum, extending from 1030 nm up to 1140 nm, whereas Nd3+ typically amplifies from 1050 nm up to 1120 nm. The result is that Yb3+ can support shorter pulse durations than Nd3+ for ultrashort pulse applications, and can also be tuned over a wider output spectrum.(e) Yb3+ has a higher saturation fluence than Nd3+, with the result that higher pulse energies can be generated by Yb3+.(f) The 920 nm and 976 nm pump bands of Yb3+ are superior to the 805 nm and 885 nm pump bands of Nd3+ from the standpoint of laser diode technology, as 900-980 nm laser diodes are currently more powerful and reliable than 800-890 nm diodes.
For these reasons, since about 1996, most of the activity in double-clad fiber lasers and amplifiers for operation in the 1030-1080 nm wavelength band has revolved around Yb3+-doped devices, and almost none has occurred in Nd3+-doped devices. The main remaining application in which Nd3+-doped fiber devices are occasionally used today is for amplification in the 920 nm wavelength band, where Yb3+ does not have a lasing transition. Yb3+-doped fibers and fiber lasers are now available for sale from numerous companies, including Nufern of East Granby, Conn., and IPG Photonics of Oxford, Mass.
Fiber lasers and amplifiers have been developed since 1996 to generate increasingly high levels of average power and, for pulsed lasers, increasingly high levels of peak power. As a result, optical power density in the fiber has increased dramatically, leading to problems with optical damage and optical nonlinearities. Fiber manufacturers have addressed these problems by increasing the diameter of the fiber core beyond the single mode cutoff. The larger core reduces nonlinearities and damage in two ways: it reduces the power density of the laser radiation, and it allows the use of a shorter fiber length, since the larger core can hold more dopant per unit length. Such fibers are typically called “Large-Mode-Area”, or LMA, double-clad fibers and are characterized by having a core V-number, V=(2πaNA)/λ, of greater than about 4.0. Herein, a is the core radius, NA is the numerical aperture, and λ is the lasing wavelength. Below the V-number of 4.0, any higher-order modes typically have very high loss.
It is often preferred for the laser output to be in a diffraction-limited beam, which requires the active optical fiber to be operated in a single spatial mode. Experimentally, it has been observed that it is possible to obtain near-single mode operation in fibers with V up to about 6.0, corresponding to a fiber core of 25 micrometers diameter and 0.08NA (numerical aperture). Such a fiber, with a typical Yb3+ doping level of ˜1% by weight, the pump wavelength of 976 nm, and the cladding diameter of 250 micrometers, can give efficient operation at fiber lengths as short as 2 m. LMA fibers have demonstrated peak output powers of over 200 kW in nanosecond pulses, albeit with severe pulse distortion and spectral broadening due to optical nonlinearities.
To achieve even higher power levels at a nearly single mode operation, the fiber diameter needs to be further increased. To reduce nonlinearities and to improve reliability, the fiber length needs to be decreased. Several techniques have been developed to increase the core diameter and shorten the fiber further, while maintaining near-single-mode operation. The common features of these fibers, henceforth referred to as Very Large Mode Area (VLMA) fibers, are the Yb3+-doped core diameter of greater than about 25 micrometers, the cladding diameter in the range from 100 micrometers to about 400 micrometers (typically determined by the brightness of the pump), and the fiber length of less than about 2 m.
One such VLMA fiber structure is disclosed by Limpert et al. in an article entitled “High-Power Rod-Type Photonic Crystal Fiber Laser”, published in Optics Express, Vol. 13, No. 4, 21 Feb. 2005, p. 1055-1058. NKT Photonics of Birkerod, Denmark, manufactures VLMA fibers of this type. In these fibers, the Yb3+-doped core is typically 40-100 micrometers in diameter, and the NA of the core is made very low (˜0.03 or less) such that all core modes other than the fundamental mode are either cut off or have very high loss, so that only the fundamental mode is propagated. The core can be defined either by a small refractive index difference between the core and cladding materials, or by a lattice of small air holes in the cladding that create an effective refractive index difference, or both. Similarly, the pump guide can be defined by either a refractive index difference or by a lattice of air holes creating an effective refractive index difference. With such a low core NA, if the fiber is bent to any significant extent, even the fundamental mode will suffer distortion and bending loss. Therefore, the fiber is fabricated with a thick layer of silica outside the pump-guiding region to give a diameter on the order of 1 mm or more, and this stiff fiber is used as a straight rod. Typically, laser form factor is an important consideration for laser users, and so a laser system that houses a straight rod fiber of length greater than about 1 m could be unattractive. However, thanks to the large core diameter in the rod-type fiber, the pump absorption coefficient for Yb3+ can be over ˜10 dB/m in such a rod fiber, and lengths of 1 m or less can give efficient pump absorption in low-gain applications.
Another VLMA technique is embodied in the chirally-coupled core fiber disclosed by Galvanauskas in U.S. Pat. No. 7,424,193. In this technique, the Yb3+-doped fiber core is a primary core made to have a secondary, smaller core wrapped helically around the primary one. The primary core has a diameter and NA selected so that the core is nominally multimode (V>4.0), but the helically wrapped secondary core is constructed to cause preferential loss for one or more of the higher-order modes, so that the fundamental mode experiences higher net gain than the other modes and thus prevails. Typically this technique would be incorporated in a double-clad structure to enable straightforward coupling of pump light at high power levels.
Yet another VLMA technique is embodied in a leakage-channel fiber disclosed by Dong et al. in U.S. Pat. No. 7,787,729. Similar to the chirally-coupled core fiber, this technique utilizes a nominally multimode core along with structural elements that cause preferential loss for one or more of the higher-order modes, and typically it would also be incorporated in a double-clad structure. In this technique, the signal light being amplified may not reside in a true fundamental mode of the structure, but rather in a so-called “leaky mode” that is confined by the structural elements in the first cladding and that remains relatively stable over the length of the device.
As expected, given the benefits of Yb3+ doping over Nd3+ as listed above, all known work using these VLMA techniques in the 1030-1080 nm wavelength band to date has used Yb3+-doped fibers. However, because the parameter space with VLMA fibers is significantly different than with previous fibers, new issues must be considered.
Specifically, Limpert et al. in an article entitled “High Repetition Rate Gigawatt Peak Power Fiber Laser Systems: Challenges, Design, and Experiment”, IEEE J. Selected Topics in Quantum Electronics, Vol. 15, January/February 2009, p. 159-169 (see section III: Gain Limitations in Short Low-NL Fibers and Consequences) discussed that in order to achieve useful levels of gain in a Yb3+-doped VLMA laser or amplifier, much higher levels of inversion must be created than in conventional fiber lasers or amplifiers. These very high inversion levels deplete the laser ground state, reducing the population of Yb3+ ions available for absorbing pump photons, thereby allowing a potentially large fraction of the pump power to travel unabsorbed through the fiber and reducing the conversion efficiency. If resonant pumping directly into the upper laser level is used, for example 976 nm pumping in Yb3+, then this effect is worsened, because some of the pump photons will stimulate downward transitions of the existing inversion instead of being absorbed. These two related problems are collectively referred to as bleaching of the pump transition. Additionally, the inventor has observed, also in agreement with other workers, that the high inversion levels can cause rapid photodarkening of Yb3+, in some cases causing the device to become inoperable within minutes.
A solution to the problem of pump bleaching, as explained by Limpert et al. in the above cited article, is to decrease the operating gain, and to increase the length of the fiber such that at the operating gain level, the fiber has adequate length to absorb the desired fraction of the input pump power. This solution has several drawbacks. First, typically the gain will be lower than it otherwise could have been, thus requiring more pre-amplification stages and thus higher cost. Second, since the pump absorption is dependent on the inversion, at low gains, all of the pump is absorbed in a short length of the fiber, while at higher inversions, not all the desired pump is absorbed, possibly causing problems with the transmitted pump light damaging downstream components. This can be particularly dangerous in transient operation, for example in an amplifier at a time between pulses, when the inversion can build up sharply and suddenly induce pump transparency in the gain medium. Third, depending on the specifics of the implementation, the additional fiber length may cause increased nonlinearities, partly undoing the benefits of using a VLMA technique. Fourth, depending on the pump configuration, there can still be regions of the fiber where the inversion is very high, and therefore photodarkening can take place in those regions.
The prior art is lacking a low-cost, low-nonlinearity solution to the problems of pump bleaching and photodarkening in VLMA active optical waveguides. It is an objective of the invention to provide such a solution.